1. Field of the Invention
The present invention relates to a semiconductor device, and particularly to improvements of an isolating region between semiconductor circuit elements and a formation method of same.
2. Description of the Background Art
FIGS. 3A-3F are cross sections illustrating processes for forming an isolating region, using conventional selective oxidation.
Referring to FIG. 3A, a SiO.sub.2 film 22 having a thickness of about 500 .ANG. is formed on a silicon substrate 21 by thermal oxidation. The SiO.sub.2 film 22 is covered with a Si.sub.3 N.sub.4 film 23 deposited to have a thickness of about 3000 .ANG. by CVD (chemical vapor deposition).
Referring to FIG. 3B, a resist layer is formed on the Si.sub.3 N.sub.4 film 23. This resist layer is patterned to form a resist pattern 24.
Referring to FIG. 3C, the Si.sub.3 N.sub.4 film 23 is etched, using the resist pattern 24 as a mask, to form a Si.sub.3 N.sub.4 pattern 23a. Thereafter, the Si.sub.3 N.sub.4 film pattern 23 and the resist pattern 24 are used as the mask for effecting ion irradiation, as shown by arrows 25, to form an impurity region 26 in a surface layer of the silicon substrate 21. In this process, if the substrate 21 is of a p.sup.- conductive type, boron ions are usually injected at a dose rate of 1.times.10.sup.13 cm.sup.-2 in an acceleration voltage ranging from 20 to 30 keV.
Referring to FIG. 3D, the resist pattern 24 has been removed.
Referring to FIG. 3E, the silicon substrate 21 is selectively oxidized thermally, using the Si.sub.3 N.sub.4 film pattern 23a as the mask to form a field oxide film 22a having a thickness of about 5000 .ANG.. In this process, oxygen is also laterally supplied by diffusion from an end of an opening in the Si.sub.3 N.sub.4 film 23a, so that a bird's beak 22b which laterally extends about 0.3-0.5 .mu.m from the field oxide film 22a is formed. During the selective oxidation, the impurity region 26 diffuses not only in a direction of depth but also in a lateral direction as shown by arrows 27 to form a channel stopper 26a laterally extending about 0.2 .mu.m beyond an edge of the bird's beak 22b.
In order to prevent activation of a parasitic MOS (metal oxide semiconductor transistor) during formation of a conductive line (not shown) on the field oxide film 22a, it is preferable for the field oxide film 22a to have a thickness as large as possible. However, the thick field oxide film 22a will increase the width of the bird's beak 22b. Therefore, in a semiconductor IC having the supply voltage of 5V, the field oxide film 22a is usually formed to have a thickness of about 5000 .ANG. so as to prevent the excessive extension of the bird's beak 22b and to set a threshold voltage of the parasitic MOS at a value of 10V or more.
Referring to FIG. 3F, the Si.sub.3 N.sub.4 film pattern 23a has been removed. Thereafter, ions are injected, as show by arrows 28, using the field oxide films 22a and 22b as the mask for forming source/drain regions 29 of, for example, the FET (field effect transistor). In this operation, the channel stopper 26a has been laterally extended beyond the edge of the bird's beak 22b and thus into the source/drain regions 29.
In order to prevent the activation of the parasitic MOS transistor, a high impurity concentration in the channel stopper 26a is preferable. However, an excessively high impurity concentration in the channel stopper 26a will reduce a junction breakdown voltage of the source/drain regions 29 contacting the channel stopper 26a. Therefore, the boron ion 25 is injected at the dose rate of about 1.times.10.sup.13 cm.sup.-2, as already described with reference to FIG. 3C.
Referring to FIG. 4A, there is illustrated a top view of an example of a memory cell array which includes the field oxide film formed by means of the selective oxidation. At an upper half in this FIG. 4A, bit lines BL are net illustrated, for simplicity reasons. FIG. 4C shows an enlarged sectional view taken along a line 4C--4C in FIG. 4A.
As seen from FIGS. 4A and 4C, each elongated semiconductor circuit element region 30 is provided with three source/drain regions 9 aligned in a lengthwise direction thereof. The three source/drain regions 9 form a pair of FETs and the middle source/drain region 9 is commonly used by these two FETs and is connected to the bit line BL through a contact hole 31. Each FET is selectively turned on and turned off by a corresponding word line WL.
In the semiconductor element regions 30 surrounded by the field oxide film 22a, the bird's beaks 22b having widths of about 0.3 .mu.m are extended along the periphery thereof, respectively, and thus effective widths of the semiconductor element region 30 are reduced.
Reference is made to FIG. 4B which illustrates an enlarged cross section taken along a line 4B--4B in FIG. 4A. The field oxide film 22a is formed on the Si substrate 21, and the bird's beaks 22b having the width of about 0.3 .mu.m is extended into the semiconductor element region 30. The channel stopper 26a, which is formed in the Si substrate and is in contact with bottom surfaces of the SiO.sub.2 films 22a and 22b, is extended about 0.2 .mu.m in width beyond the edge of the bird's beak 22b into the semiconductor element region 30. Between the opposite edges of the bird's beaks 22b, there is formed a gate oxide film 32 on the Si substrate 21, and a word line WL is formed on the gate oxide film 32.
As apparent from FIGS. 4A and 4B, if the semiconductor element region 30 has a width of 1 .mu.m or more, the semiconductor element region is not completely covered with the bird's beak 22b, and the channel stopper 26a does not extend into the entire area of the semiconductor element region 30. That is, if the semiconductor element region 30 has the width of 1 .mu.m or more, there remains a region or area for forming a FET inside the channel stopper 26a and the bird's beak 22a extending inwardly from the periphery of the semiconductor element region 30.
However, if the width of the semiconductor region 30 becomes as small as 1 .mu.m, the effective region for forming the FET is narrowed, which reduces a current value of the FET and increases a contact resistance of the contact hole 31 resulting in deterioration of the performance of the semiconductor IC. Particularly, in a small FET having a channel less that 1 .mu.m, a so-called short-channel effect in which a threshold voltage fluctuates, is caused due to intrusion of the channel stopper 26a into the source/drain regions.
Further, in the semiconductor element region 30 having the width less than 1 .mu.m, the channel stopper 26 extends throughout the semiconductor element region 30, which makes the formation of the FET difficult. Moreover, if the width of the semiconductor element region 30 is reduced to be less than 0.6 .mu.m, the semiconductor element region 30 is entirely covered with the bird's beak 22b, which makes the formation of the FET impossible.
Referring to FIG. 5, there is illustrated simultaneous formation of a channel stopper layer and a punch-through prevention layer by ion implantation through a field oxide film pattern according to the prior art. For example, boron ions 35 are implanted with an acceleration energy of 200 keV through a field oxide film pattern 22a, 22b having a thickness of 5000 .ANG.. As a result, a channel stopper layer 36a and a punch-through prevention layer 36b are formed simultaneously.
However, the field oxide film pattern includes bird's beak 22b, the thickness of which decreases gradually, and thus a transition impurity layer 36C is formed between the channel stopper layer 36a and the punch-through prevention layer 36b. During a heat treatment at a later stage, the transition impurity layer 36C is liable to spread undesirably by diffusion into the semiconductor device region neighboring the field oxide film 22a, 22b.